What are the benefits of lactic acid for the skin?

Lactic acid is part of the alpha-hydroxy acid (AHA) family, a class of molecules renowned for their ability to exfoliate the skin. Unlike mechanical scrubs with granules, which act by friction, lactic acid works chemically by breaking the bonds between dead cells in the stratum corneum, thereby facilitating their gradual detachment from the epidermis. As an exfoliant, lactic acid is used at concentrations of 10% or less. What sets lactic acid apart from other AHAs, such as glycolic acid, is its relatively high molecular weight (90.08 g/mol for lactic acid versus 76.05 g/mol for glycolic acid). This characteristic of lactic acid limits its penetration into the epidermis, making it an exfoliant that is generally well tolerated by sensitive skin.

Good to know : Regular exfoliation of the face promotes a smoother skin texture, a more luminous complexion and minimises pore obstruction.

The lactic acid does not limit itself to an exfoliating function: it also contributes to skin hydration and the reinforcement of the skin barrier. Several studies have demonstrated that it stimulates ceramide biosynthesis in keratinocytes, essential lipids for maintaining the cohesion of the stratum corneum and whose organisation is closely linked to transepidermal water loss. In vitro, application of L-lactic acid led to an almost 300% increase in ceramide levels in epidermal cells. These results were confirmed by isotopic labelling experiments, demonstrating that lactic acid could be metabolised by the cells and used as a carbon source for lipid synthesis.

The beneficial effects of lactic acid on the skin barrier have also been observed in vivo, in a four-week, double-blind clinical study involving 24 volunteers. The twice-daily application of a lotion containing 4% L-lactic acid not only enhanced the lipid barrier and reduced skin damage induced by the sodium lauryl sulphate, but it also demonstrated, according to a clinical evaluation conducted by a dermatologist, a marked reduction in signs of skin xerosis. These observations confirm that L-lactic acid acts as a barrier booster, combining moisturising action with lasting protection against dryness.

Lactic acid is also credited with the ability to delay the visible signs of skin ageing. Indeed, studies in vitro have shown that this molecule can stimulate collagen production by fibroblasts. As a reminder, collagen is a dermal protein that contributes to the skin’s suppleness and firmness. This potentially tightening effect of lactic acid is accompanied by an antioxidant activity that enables it to neutralise free radicals before they attack cellular components. Generated during energy metabolism, but also following exposure to UV radiation, pollution and tobacco, these free radicals are among the primary culprits in accelerated skin ageing.

Coupled with its ability to stimulate cell renewal, these various mechanisms explain why lactic acid is frequently used in professional peels to reduce wrinkles.

Lactic acid is also of interest for evening out the complexion. Like other acids in the alpha hydroxy acid (AHA) family, it can inhibit tyrosinase activity, an enzyme involved in melanogenesis, the process of melanin production. Tyrosinase specifically catalyses the conversion of tyrosine, an amino acid, into melanin. A study on mouse melanoma cells showed that the inhibition of tyrosinase by lactic acid was dose-dependent and comparable to that of glycolic acid.

Furthermore, the exfoliating action of lactic acid also contributes to attenuating pigmentation spots. Indeed, by favouring the desquamation of the epidermis, this compound gradually disperses melanin clusters situated in the basal layer. This process partly relies on the chelation of calcium ions, which weakens the junctions between corneocytes and reduces their adhesion, thus facilitating cell renewal. The combination of these two mechanisms—inhibition of melanogenesis and acceleration of epidermal renewal—explains why lactic acid is recognised for its efficacy against pigmentary irregularities and brown spots.

Lactic acid also plays an interesting role as an antibacterial agent. Studies have shown that at a concentration as low as 0.5%, it can completely inhibit the growth of pathogens such as Salmonella enteritidis, Escherichia coli or even Listeria monocytogenes. Its mode of action is primarily based on a profound alteration of the bacterial membrane: by penetrating the periplasmic space via the channels of the outer membrane, lactic acid interacts with the lipopolysaccharides and phosphate groups of the membrane, altering their charges and weakening the interactions that maintain its stability. This disruption favours the release of essential components, such as lipopolysaccharides and glycerophospholipids, leading to a loss of membrane integrity. Consequently, intracellular contents gradually leak out, causing sublethal damage and ultimately resulting in complete inhibition of bacterial growth.

When applied to the skin, this antibacterial mechanism makes lactic acid particularly valuable for preserving the balance of the cutaneous microbiota, thus preventing the risk of dysbiosis, notably acne.

Lactic acid is also endowed with anti-inflammatory and immunomodulatory effects which account for its soothing role. At high concentrations and in an acidic environment, it reduces the production of pro-inflammatory cytokines such as TNF-α and IL-1β by macrophages and monocytes. Several studies have shown that it can interfere with monocyte energy metabolism by inhibiting glycolysis, thereby limiting their differentiation into dendritic cells and reducing T-lymphocyte activation. It has also been demonstrated that lactic acid delays the expression of genes activated by LPS, notably those encoding the chemokines CCL2 and CCL7. This mechanism is linked to attenuated phosphorylation of protein kinase B (PKB) and decreased degradation of IκBα, thus reducing activation of the transcription factor NF-κB, a key mediator of inflammation.

Applied to the skin, these anti-inflammatory effects of lactic acid could help to soothe redness and cutaneous hypersensitivity. By limiting the inflammatory cascade, it may be beneficial for skin prone to inflammatory dermatoses, such as eczema, rosacea or inflammatory acne.

The exfoliating, antibacterial and anti-inflammatory properties of lactic acid, previously discussed, may be particularly relevant in the context of blemish-prone skin. Indeed, a clinical study evaluated the efficacy of a series of 88% lactic acid peels in 25 patients with skin phototypes III to VI suffering from acne, 15 of whom also had acne scars. Three sessions spaced two weeks apart were performed, with a follow-up three months after the end of the protocol. The results showed a significant reduction in inflammatory lesions, as shown in the table below. Regarding scarring, improvement was moderate in 20% of patients, marked in 20% and excellent in 60%. Despite some cases of transient post-inflammatory hyperpigmentation, this resolved within three months, resulting in a more even skin tone.

Finally, lactic acid can support the cutaneous repair process. As early as the 1960s, it was shown that lactate stimulates collagen synthesis by fibroblasts, notably via the activation of prolyl hydroxylase, an enzyme essential to collagen fibre maturation. More recently, studies have confirmed that lactate acts as a genuine regeneration signal, promoting endothelial cell migration and the recruitment of vascular precursors. These effects converge to stimulate angiogenesis, that is to say the formation of new blood vessels, thereby ensuring an improved supply of oxygen and nutrients to regenerating tissues.

Murine models have also shown that lactate enhances extracellular matrix deposition and accelerates cutaneous wound repair. One notable example is a study conducted on rats bearing wounds. The animals were divided into two groups, one receiving a daily application of a cream containing 50 mM sodium L-lactate, the other the vehicle alone. The results showed that lactate application significantly promoted wound healing, by increasing vascular density at the wound site and reducing lesion diameter over time. These data reinforce the idea that lactic acid and its derivatives can accelerate skin healing by supporting tissue regeneration and neovascularisation.

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